I bought a Litokala 500 engineer from Alibaba for $14 shipped, taking one of them apart might be instructive. The insides of one of the USB doctor measurement devices might be useful too.

Mine arrived yesterday. They sanded most of the part numbers.. I’m not going to post any images. There are a couple of videos on YouTube if you are curious.

It is about what you would expect. A micro controller for the display board and another one on the mother board. There are a few power transistors and chokes. The USB power out circuit is at the top by the jacks.

I made two design choices yesterday. Both are open for discussion. It is easy enough to undo this early in the design.

The LTC4015 can reduce charge current if Vin drops below a hardware set threshold. Typical use would be when Vin also powers a device. If the device starts drawing more power, the charger will throttle back. In this application, it would be used if we program a current that is too big for the power supply.

I decided not to implement under voltage current limiting. I will leave it up to the end user to source a suitable power supply. Leaving it out simplifies the board.

Cell count is set in hardware by three pins. The LTC4015 does not monitor individual cells and cannot balance a series string. I’m leaning towards hard wiring this to one cell only.

The other option would be to tie Cells 2 low with jumper selection on Cells 0 and 1. That would allow 1,2,3…6 all the way to 9 cells in series. 3 and 6 would be for 6 and 12V lead acid. If I do that, I’ll design for 15 or 20V Vin Max and 8A I charge max. The higher current would make this more useful for Pb batteries.

LOVE THIS STUFF!!! Alot of my ideas never make it much further than design either. I’ve been wanting to learn USB/Charging circuitry for awhile and this is !!!

Thank you so much for sharing!!!

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Should my information be incorrect in the slightest please inform me as i am always learning as well and may not recollect everything properly. ALSO, ANYTHING I WRITE PLEASE TAKE WITH A THOUSAND GRAINS OF SALT AND YOUR OWN LIABILITY, AS YOU SHOULD WITH ALL INFORMATION.

Andrew_Debbie, I believe the people here is more concerned with what this can do and not so much with any particular optimizations. I'd stick a BIG ASS inductor in there and let users enjoy unprecedented charging flexibility and features (like fully adjustable charge voltage).

Added a few more parts. The Screw terminals at Vin are rated 15A nominal. Should be fine at 8.

R4 is a 10k NTC thermistor. It will end up mounted off board on Rev 0. On the schematic now mostly for BOM. More decisions later. I’m thinking about sacrificing a $4.99 charger for the battery holders.

R7 and R8 are the current sense resistors. The 4015 sets the current by measuring the voltage between CSP and CSN in 1mv steps up to 32mV. Maximum current is at 32mV. 8mΩ == 4Amps and 4mΩ gives 8A.

I’m going to set this up for 2 resistors in parallel. Put both of them in if you want 8A. Only put in 1 for 4A. — Of course you can use different values… The resistors are going to dissipate about 1/8 W. I’m going to use 2 or 3W resistors so that they don’t heat up too much.

CSN and BATSENS both connect to the battery positive terminal. At 8A or even 4A there could be enough voltage drop across the wire from CSN to the battery to matter. I haven’t checked. I’ll also have to model the voltage drop on the negative side. All of that will happen later on, when I start too look at layout and packaging.

There are people on here that have a lot more experience with small power FETs than I do. I'm happy to have suggestions. There are lot of things to check. The main one is to not over load / over heat the internal gate drivers and the LDO inside the LTC4015. it is possible to use an external LDO to power the gate drivers.

LT's example designs use an On-Semi (Fairchild) FDMC8030 Dual N-Channel MOSFET for the top and bottom switch. The FDMC has good numbers -- rDS on < 14mΩ at 10A , VGS = 4.5V

The FDMC8030 packs two FETs into a tiny 3x3mm MLP package. I like the specs but not the challenge of getting the layout right or being able to solder the part.

Going through the product selector at On-Semi, I found a couple of older Fairchild parts that should work.

FDS 8949 Dual N-Channel At VGS = 4.5V rDS on = 36mΩ 6A.

I'm not sure if this one will work at 8A. Should be ok at 4A but I'd have to do some thermal estimates.

FDS 8447 Single N-Channel At VGS = 4.5V it is 12.3mΩ and 11.4A.

It is single transistor. Trade off of size for better specs and easier layout.

The battery is a dummy to show the complete circuit. There should be a capacitor in parallel with the battery.

I’ve left out the FETs that are supposed to switch between Vin or battery power for the load. SYS is suppose to be the greater of Vin or Vbat. Tying SYS to VIN without a switch could be a problem if you pull the power with a battery installed. I may have to add two FETs. I may do it anyway.

I did first order sizing on the inductor and some of the capacitors. The data sheet has equations and recommendations. I haven’t done any thermal calculations yet.